As an electric motor to drive a fan of an air blower, an air conditioner, or the like, a synchronous motor in which permanent magnets are used in a rotor is often used to reduce a power consumption during an operation. Such a synchronous motor using the permanent magnets is typically driven by a square-wave or sine-wave drive system that detects the magnetic pole position of the rotor and causes a drive current synchronized with the magnetic pole position of the rotor to flow through the stator winding by using an inverter including semiconductor elements or electronic parts. For example, with the 120°-square-wave drive system that is used for driving a three-phase synchronous motor, the cycle of 120° forward energization, 60° non-energization, 120° reverse energization, and 60° non-energization is repeated for respective phases at the timings shifted by 60° from each other, so that a current flows through the stator windings of any two of the three phases to drive the motor. Such a square-wave drive system can be achieved with a simple control mechanism compared to that for a sine-wave drive system, which contributes to the cost reduction.
Also, in a three-phase synchronous motor in which stator winding wound around teeth in a concentrated manner, the ratio of the number of magnetic poles of the permanent magnet used in a rotor and the number of slots (=number of teeth) in a stator is often 2:3. Also, one known synchronous motor uses a combination of the number of magnetic poles and the number of slots that enables a magnetic flux generated by permanent magnets to be effectively interlinked with a winding of a stator (see, for example, Patent Literature 1 and 2). A synchronous motor using such a combination of the number of magnetic poles and the number of slots is a highly efficient synchronous motor having a small loss therein.